(1) Field of the Invention
This invention relates to an optical recording medium permitting the reproduction of recorded information such as images, voice or sound by laser beam, and more particularly to an optical recording medium permitting the recording of such information by exposure to ultraviolet rays.
(2) Description of the Prior Art
Reflecting recent advancements in laser technology, optical recording making use of laser beams for the recording and reproduction of information has been attracting more and more attention. It is already possible to reproduce information on video disks or audio disks. This optical recording method provides merits that conventional analog recording methods, e.g., phonographic records or conventional magnetic recording methods, are unable to achieve: i.e., the optical recording method permits a large recording density, assures long service life owing to its non-contact reproduction, and features short random access time.
In recording media such as video disks and audio disks, recording pits of 1 .mu.m or smaller are formed by mechanical methods such as injection molding or press forming. These conventional methods are however accompanied by many problems from the standpoint of production engineering, which problems include the need for expensive precision molding machines, limitations in materials as the molding materials and the clogging or loading of pit-transferring stamps.
Researchers of the present assignee company conducted investigations to overcome such problems and to develop an optical recording medium which may be produced with a high production yield by means of a simple apparatus. They have already proposed novel optical recording systems and media as disclosed in Japanese Patent Laid-open Nos. 195336/1982 and 195341/1982.
The above optical recording system and medium permit the reproduction of records by a laser beam having an oscillatory wavelength in the visible or near infrared wavelength range while making use of the fact that, when a desired area of a recording layer containing a compound having an absorption maximum in a specific visible or near infrared wavelength range is exposed to radiation such as ultraviolet rays or electron beams, the optical absrotpion in the visible or near infrared wavelength range is diminished. Accordingly, the above-proposed system and meidum are exceptionally good because they do not require the formation of pits by a mechanical method such as injection molding.
When radiation is used to record in the above-proposed optical recording system, the diminution of optical absorption of wavelengths in the visible to near infrared range is not always sufficient if the radiation has ordinary energy. In other words, the above optical recording system requires radiation, for example, having an intensity as high as at least 500 mJ. Therefore, there has been a standing demand for the development of a recording medium which permits recording with low radiation energy.